The prior art discloses a variety of methods and apparatus which produce solid material directly from a source of molten material. Most prior art systems are for the fabrication of metal products and use some type of fixed, rigid, noncontrollable forming orifice to stabilize the dimensions of the product.
A typical system of this type is illustrated in U.S. Pat. No. 2,825,108 to Conn in which molten metal is made into a filamentary form by forcing it through an orifice so as to generate a free standing stream of molten material which is subsequently solidified into filamentary forming on a rotating, heat extracting member. The rate of production is determined by the rate at which the molten material is expelled from the orifice and for continuous filament this rate must be approximately synchronous with the rate of movement of the heat extracting member at its point of contact with the filament.
Techniques of this type are troubled by the relative complexity of the necessary process control system and the difficulty in passing a molten material through fixed, small orifices. The orifices must be constructed from an exotic material if the molten material has a relatively high melting point. The orifices have a tendency to erode and/or become partially or completely blocked due to the deposit of material on the orifice.
An improvement is disclosed in U.S. Pat. No. 3,838,185 to Maringer et al. That patent discloses a disk-like heat extracting substrate which is rotated and lowered into the upper surface of a molten material. The peripheral edge of the disk-like substrate moves generally parallel to the surface of the molten material. More specifically, the edge moves slightly downwardly through the surface of the melt into a region of contact and then moves upwardly and out of the melt carrying a layer of molten material which is then chilled as the wheel rotates and carries it away from the surface of the molten material. After the layer is solidified, it is removed from the substrate in the form of a filamentary product without the use of forming orifices.
One difficulty with this system, however, is that, because the heat extracting substrate is inserted downwardly into the surface of the melt, the melt surface must be opened and exposed to the atmosphere. This exposure to the atmosphere is a major source of heat loss and therefore increases the energy required to maintain the molten material surface at a suitably high temperature for proper operation of the system. Second, the exposure of the molten material to the atmosphere provides a source of contamination which may be introduced in to the molten material from the atmosphere, permits the escape of relatively voltatile constituents from the molten material and allow oxidation reactions to occur at the surface of the melt. It is therefore difficult to maintain a constant chemical composition in the molten material. Further, this system permits limited control because the region of contact of the melt with the substrate must always be at the lower-most segment of the substrate. Thus, control of the process is limited to control of the substrate speed, depth of penetration of the substrate into the surface of the molten material and temperature.
A second system for overcoming some of these objectionable characteristics is disclosed in U.S. Pat. No. 3,896,203 to Maringer et al. In this system a melted drop of material is adhered to a solid member by means of the molecular attraction of the molten material for the solid member to form an adherent pendant drop. The drop is suspended from the solid member. The drop remains adherent because the net molecular attraction force is greater than the net gravitational force on the drop. The adherent pendant drop is then contacted by a rotating heat-extracting member. The pendant drop may be formed by locally heating the end of a solid member of the material to melt the end. Alternatively, the pendant may be formed, adhered to and suspended from an orifice through which melt is forced, for example by the pressure head exerted by a height of molten material above the orifice.
A variety of difficulties have been encountered in connection with the pendant drop system. A component of the shear forces exerted on the adhered drop by the rotating heat extracting member adds to the gravitational forces and together they tend to detach the drop from the solid member. Furthermore, because the drop was suspended from the solid member by gravity, the rotating substrate normally was beneath or partly beneath the drop. Upon contact of the molten material with the heat-extracting substrate, gravitational forces acted in a direction to move the molten material onto the entire surface of the rotating substrate. If the substrate was of the type having a plurality of raised ridges on its peripheral surface for forming filamentary products on the crests of these ridges, the melt would also flow by gravity down in between the ridges to form ribbons or sheets. Further, if the pendant drop is formed at the end of a piece of solid material from which the droplet is formed by local heating, then an additional manufacturing step is needed. The raw material must be formed into solid rods or other geometrical shapes and then later locally melt to form the drop. This is not only an extra production step but requires additional energy.
It is therefore an object of the present invention to provide a system for forming ribbon, filaments, fiber or film products directly from a molten material which will not require that the top surface of the molten material be exposed to the atmosphere, which will not permit the shear forces exerted on the melt at the region of contact between the heat-extracting or chilled substrate and the melt to have a tendency to detach a large mass of the melt from the main body of molten material and which additionally will permit the molten material to be formed by a single melting step in which the component raw materials are melted in a receptacle and used directly to form the resulting products.
In addition to providing the combination of these three advantages, it is a further object of the present invention to provide a system which overcomes forces which tend to urge the molten material to fall into the valleys between a plurality of ridges formed on a cylindrical, rotating, chilled substrate.
Yet another object of the present invention is to provide a system in which the substrate can be moved at a significant velocity past a small confined region of contact with the molten material without materially disturbing the stability of the pool of the molten material.
Other objects and features of the invention are described below or will become apparent from the following description.